The species-rich class of substances known as cellulose ethers, including the group of binary alkylhydroxyalkyl celluloses containing the commercially utilized representative methylhydroxypropyl cellulose (MHPC), has been studied for many decades in university and industrial spheres of activity and has been described many times. A review of the chemical background and principles of preparation (methods of preparation and process steps) and a material survey and description of the properties and opportunities for application of the various derivatives of cellulose ethers is given, for example, in Houben-Weyl, Methoden der Organischen Chemie, Makromolekulare Stoffe, 4th edition, vol. E 20, p. 2042 (1987).
The methods described and used for preparing alkylhydroxyalkyl celluloses, such as methylhydroxypropyl cellulose, are based on either heterogeneous (multiphase mixture of substances) or homogeneous (e.g., single-phase solution) reaction management. The process itself may be performed as either a batchwise or a continuous procedure. Furthermore, in the case of a heterogeneous mode of reaction, the so-called gas phase process (without a fluid reaction medium) is differentiated from the so-called slurry process (in the presence of a fluid reaction medium).
All the process variants described and industrially utilized for preparing alkylhydroxyalkyl celluloses, e.g., methylhydroxyalkyl cellulose, are based on the following chemical principle of reaction. In a preliminary sub-step, the cellulose starting material is activated, preferably with a caustic alkali solution. Then, the alkali metal cellulose formed is force-reacted with the corresponding alkylene oxide and methyl chloride, wherein any optionally used excess alkali is expediently neutralised with more than the stoichiometric amount of methyl chloride. In the subsequent purification step, the salt formed and any other secondary products are removed, preferably by washing with hot water.
DE-A 2402740, U.S. Pat. No. 2,949,452 and EP-B 134465 describe so-called gas phase processes for preparing MHPC in which no fluid or condensed media are present during the etherification reaction. The substitution (DS and MS values) can be varied over a wide range when using this process. Due to the lack of a fluid heat transfer medium, however, the exothermic character of the chemical reaction can be controlled only inadequately; in addition, there are problems with distributing the alkali and reactants used. All in all, this is expressed by only moderate reproducibility of the substitution and uncontrolled and relatively severe degradation of the molecular weight and thus results in a variable set of properties for the product. Moreover, highly viscous products are not obtainable via the gas phase process due to severe degradation of the molecular weight.
The problems mentioned when discussing the gas phase process occur to a much smaller extent, if at all, in the presence of a fluid reaction medium. Thus, in the so-called slurry process, inert organic solvents, the reactant methyl chloride in excess, or an appropriate mixture thereof, normally function as a distribution medium and heat transfer medium. The reaction medium present during the activation and reaction phases brings about, on the one hand, more uniform substitution with higher reproducibility and higher chemical yields, due to more uniform alkalisation of the cellulose and better transport of the reactants into the alkali metal cellulose. On the other hand, the process is altogether more controllable as a result of effective heat dissipation and degradation of the molecular weight is clearly suppressed due to the avoidance of localised overheating, so very highly viscous products are also obtainable. Due to these chemical engineering and product property advantages, the method of preparation used on an industrial scale has largely evolved as a slurry process.
The disadvantage of the standard slurry process, in which the entire amounts of the reactants alkylene oxide and methyl chloride are present alongside each other during the etherification phase, is that the degree of substitution MS can be adjusted to only a limited extent. For example, in the case of methylhydroxypropyl cellulose, exclusively products with a high DS (methyl) and a low MS (hydroxypropyl) are produced by formal parallel reaction of the reactants. The reverse product variants, i.e., high MS (hydroxypropyl) and average to low DS (methyl), are not obtainable using this type of procedure for reaction kinetic reasons, even when increasing the amount of propylene oxide used. The aforementioned highly propoxylated MC derivatives, however, are of interest due to a number of substance-specific properties.
According to U.S. Pat. No. 4,096,325, highly propoxylated MC derivatives can be prepared if the hydroxypropylation and methylation are performed in a manner which keeps them largely separated. EP-A 567869, for example, describes, as a process of just this type, the stepwise reaction of the alkali metal cellulose generated, initially with propylene oxide and then, in a solvent, with methyl chloride. The DS and MS values can be varied over a wide range in this way.
In the case of process variants specified with stepped reaction management, hydroxypropylation is generally performed at high temperatures. On the other hand, methylation, which is a reaction which proceeds exothermally per se, is performed with counter-cooling at relatively low temperatures. Due to the long process times and the counter-flow of energy, this process is less suitable for economically viable large-scale production. In addition, with increasing separation of the reaction steps, in the same way as in the gas phase reaction, problems occur with regard to the uniformity and reproducibility of substitution, temperature management and molecular weight degradation (final viscosity).
Due to the disadvantages mentioned in the preparative variants which have hitherto been developed and described, there is a constant demand for a process which provides both the chemical engineering and product-specific, and economic, advantages of a slurry process along with the flexibility involved in a gas phase process with regard to obtaining yields and arrangements of the relative proportions of the degrees of substitution MS and DS over a wide range.
The background to the present invention was therefore the provision of a process for preparing alkylhydroxyalkyl celluloses, such as methylhydroxyethyl cellulose and methylhydroxypropyl cellulose, which facilitates, with high reproducibility and chemical yields, a wide variation in the degrees of substitution MS and DS and also the product viscosity.